fiber: leak slab if unable to bring prots back

In case if we unable to revert guard page back to
read|write we should never use such slab again.

Initially I thought of just put panic here and
exit but it is too destructive. I think better
print an error and continue. If node admin ignore
this message then one moment at future there won't
be slab left for use and creating new fibers get
prohibited.

In future (hopefully near one) we plan to drop
guard pages to prevent VMA fracturing and use
stack marks instead.

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>